So Face ID will allow anyone to grab your phone and hold your head in the correct position and open your phone instantly. With a decent passcode it will take them quite some time to try every combination, maybe days or longer. So seems to me it makes your phone less secure. Kind of like those building access locks that use a thumb print. The crooks will just have to take your Thumb with then then.
While not being an expert in law, I do believe that would come with a much heavier charge. Also, there is the practical reality of it... Not as easy as all that to cut someones digits off. Takes some kind of psychopath/sociapath that likely is already doing heavier crime then stealing phones.
IF this was a thing that was happening, we would already know of it. It would be all over the news. Are you kidding me? iPhone users getting their finger cut off would be headline sugar the news outlets would kill for.
Considering that iPhones are easy to locate after getting stolen.. A crime such as that would be high priority to pursue and police would be using "Find My iPhone" on the regular to catch criminals...finally.
Which is why this is just not a thing in reality.